My hat's off to Primary
Energy (hat tip:
Knowledge
Problem). Years ago they
took almost all of the concepts for generation-side efficiency improvment
and cogeneration that I outlined in
Where
to go from here? and are promoting, commercializing and installing
them on the industrial side. I am a latecomer to this party.
I should have discovered them early on during what I was calling
research. They slipped under my radar despite their field being
exactly in my area of focus. (I have wondered out loud why so
many seemingly-obvious opportunities for savings seem to have gone
unrecognized and unused. Primary Energy has recognized them and
is doing something about it.)

I could take this as a personal failing on my part, or as an indication
that Primary Energy is not getting nearly enough press. That press
just got a whole lot broader with an article in "Skeptical Inquirer"
(a magazine not typically noted for commentary on energy matters), and
notices in other blogs. Here's a bit more press for them; I hope
it helps.

The SI article is
newly available on-line, and all of the salient points (and more)
are contained between
this Distributed Energy article and
this presentation. (I can console myself with the fact
that many of these articles and presentations appeared after I did the
majority of my research.) The interested reader should take time
out and read at least one of those pieces before continuing. Go
ahead, take your time; this page isn't going to be impatient with you.

Done? Good.

Primary Energy estimates that industrial energy recycling could yield
between 40 GW and 100 GW (page 10), of which only 2.2 GW are currently
being used. My own estimate of the cogeneration potential of
natural-gas fired residential and commercial space- and water-heating
requirements is
50 GW average; this potential is almost completely unused.
This adds up to between 88 and 148 GW of untouched electric generation
potential which can be obtained with zero (flare gas, heat recycling,
pressure drop) to minimal (residential and commercial cogeneration)
additional fuel use. This could meet 1/3 to 1/2 the additional
electric demand created by a complete conversion to plug-in hybrids.

I'm more of a physics/thermodynamics wonk rather than a regulation wonk (though I could have told you the effect of California's highly-regulated "deregulation"), so I don't really have many opinions on such matters. If you want to deal with people who are into power from the legal/regulatory end you might be better off hanging out at Knowledge Problem.

I don't know of any, and based on the size vs. efficiency relationship for gas turbines (viscous and conduction losses rise rapidly as size goes down) I doubt that a house-sized turbine could beat an Otto- or Diesel-cycle variant.

Which is not to say that nobody has tried building a home-sized gas turbine, even wood-fired. The feasibility of this concept is TBD.

Recent reader here -- wondering whether residential cogeneration is actually feasible. Unlike power cogeneration -- where the processes are approximately constant -- it seems to me that the use of gas for residential purposes is too inconsistent and diffuse to actually use for cogeneration. For instance: would it be feasible to capture waste heat from cooking? From water heating? From space heating? None of these are going all the time, and they happen in different parts of the house.

Quite. AAMOF, there have been some hobbyists doing it for years, and some unsuccessful attempts at commercializing residential cogeneerators. All it takes is a big enough spread between the cost of heating fuel and the cost of electricity.

it seems to me that the use of gas for residential purposes is too inconsistent and diffuse to actually use for cogeneration.

This doesn't matter much. Individual homes may have irregular usage patterns, but the aggregate of a neighborhood (let alone a city) will be very smooth and predictable. In the case of on-grid users, the averaging will handle things; off-grid users are already using batteries and will use them to balance supply and demand.

would it be feasible to capture waste heat from cooking?

Probably not for electrical generation purposes, though it might be possible to use the waste heat from a gas-fired generator of some kind for cooking. Solid-oxide fuel cells are the candidates I'm thinking about; they run more than hot enough to heat ovens, and maybe hot enough to use as burners.

From water heating? From space heating?

You're thinking backwards here. The waste heat from the cogenerator would be used to heat water and air, rather than using space heat to make electricity.

None of these are going all the time, and they happen in different parts of the house.

In all the houses I've lived in, the generation of space heat and DHW happened within a few feet of each other. This would make it relatively easy to supply the two heat demands from a common cogenerator, and feed excess electricity to the grid. It's true that heat demand and electric demand don't always match, but you could finesse this somewhat by e.g. supplying the morning shower from stored hot water and not reheating it until afternoon, when the electric demand peak hits.

A user with net metering and time-of-day pricing might be able to pay for their hot water using such a scheme. If natural gas is running less than $1.00/therm (3.4 cents/KWH) and electricity costs $0.15/KWH or more on the afternoon peak, a cogenerator could gross 11.6 cents/KWH on the difference. On an 80-gallon tank of water heated from 40 F to 120 F (approximately 52,000 BTU) that would be roughly 65 cents of natural gas to heat the water and yield roughly 3.8 KWH (57 cents worth) of electricity (assuming a cogenerator which produces 20% electricity and 80% heat). Reducing the cost of hot water from 52 cents/tank to 8 cents/tank isn't bad, and the system would yield a profit (before maintenance and amortization) at 18 cents/KWH.

There are two problems with this: making and installing enough systems that the cost of buying and running one is reasonable, and making the benefits understandable to enough people that the necessary volumes are possible. I'm trying to help with the understanding part, though I know I'm not the best at writing essays which are accessible to people who have not taken a course in thermodynamics (most of society).